This invention relates to miniature telescoping spring loaded electrical probes (hereinafter telescoping spring probes) for establishing low resistance eclectrical test connections with conductors on, for example, printed circuit boards.
Telescoping spring probes are generally known for making electrical test connections with conductors on printed circuit boards. They are generally affixed in a jig employed in a circuit tester, and continuity, lack of continuity or other electrical characteristics between the various electrical nodes on a printed circuit board are detected through the probes.
Modern circuit testers generally require telescoping spring probes for making electrical contact through each probe to a different one of a number of very closely spaced nodes. Typically it is necessary in each jig to make electrical contact with, for example, hundreds of nodes, any one of which may be spaced as closely as 0.100 inch on center. As a result, a need has arisen for spring probes with an outside housing diameter (O.D) in the order of 0.04 to 0.150 inch. The problem is made more complex by the requirement that consistent low electrical resistance be maintained through the telescoping spring probe.
Because of the extremely large number of telescoping spring probes required for each jig and circuit tester, the total cost for the telescoping spring probes may be quite substantial and even a minor increase in cost for each probe results in a very substantial increase in total cost for the overall circuit tester.
One type of telescoping spring probe is known and is commonly referred to as the snap out spring probe. In this device there is a receptacle and an interchangeable spring probe which is inserted into the receptacle. The receptacle in turn is inserted through a nonconductive mounting member so that the bottom end of the receptacle is exposed for crimping, wirewrapping, soldering, etc., to a conductor. The spring probe includes an elongated electrically conductive tubular housing and an elongated electrically conductive plunger telescoped in the tubular housing. The plunger has a probe head exposed at one end of the tubular housing. A spiral compression spring mounted inside of the tubular housing urges the plunger and housing to a fully extended stop. Electrical connection is made from the probe head to the bottom end of the receptacle via the connection between the plunger and the inside wall of the housing and through the housing to the inside wall of the receptacle.
It has been found that the electrical resistance between the plunger and housing varies with prolonged cycling, due to wear of the parts, wear particle buildup between parts, movement between parts, and changes in pressure of the coil compression spring during a telescoping cycle. As a result, attempts have been made to reduce the resistance between the plunger and tubular housing by the introduction of small metal balls between the plunger and wall of the tubular housing and by the addition of separate wiper springs between the plunger and housing. However, these special parts result in added cost not only in parts but in labor.
Electrical connectors are known which eliminate the need for connection through the housing and receptacle. For example, a connection may be made directly to the tubular housing itself, eliminating the receptacle. By way of additional example, U.S. Pat. No. 3,437,984 discloses an electrical spring probe having a conductive plunger with an internal end in a housing and an external probe end. The internal end engages a conductive spiral compression spring. The spiral compression spring serves to urge the plunger to a fully extended position and its opposite end is externally exposed for connection to a conductor. Electrical connection is formed through the plunger and the spring.
Another prior art device involves a straight through probe having an elongated tubular housing and an elongated conductive plunger which extends completely through the housing. The plunger has its opposite ends exposed, one end comprising a probe head and the opposite end comprising a connector to which a conductor may be wirewrapped, crimped, soldered, or otherwise connected. A spiral compression spring urges the plunger and housing to a fully extended stop condition.
In all of the aforementioned telescoping spring probes the tubular housing is generally formed from a solid tube and generally a stop in the form of a detent or enlargement is formed on the plunger. Since it is necessary for the spiral compression spring to preload the plunger, it is necessary to retain the preloaded plunger while forming a stop in the tubular member to limit travel of the plunger relative to the housing. In this regard the stop may be formed in the housing by forming a detent or rolling an inwardly extended ring around the perimeter of the housing.
These steps add substantially to the cost of manufacturing a telescoping spring probe.